A method of measuring the tear film thickness is described in which interference causes oscillations in the reflectance spectrum from the tears. Strong oscillations were usually observed when a contact lens was worn. Measurement of modulation and phase of these oscillations confirmed that they were associated with the tear layer in front of the contact lens. Calculated thickness of this layer averaged 2.7 microns. In one out of five subjects, weak oscillations were sometimes observed without a contact lens. These oscillations probably arose from the aqueous layer of the tears with a thickness of approximately 3 microns. The relative merits of three interference methods of measuring the tear film are discussed.